Chair backrest



FIG. 1 is a front perspective view thereof;

FIG. 2 is a front view thereof;

FIG. 3 is a rear view thereof;

FIG. 4 is a right side view thereof;

FIG. 5 is a left side view thereof; and,

FIG. 6 is a top view thereof.

Features shown in dashed lines form no part of the claimed design.

The ornamental design for a chair backrest, as shown and described.